In an integrated circuit (IC), it is not uncommon for both high and low voltage devices to be included on the same IC. For example, the IC may include 1.5V digital core device and 6V high voltage devices. The supply voltages for such 1.5V and 6V devices are 0.95V (DVDD) and 3.4V (VH), respectively.
Level shifters are used to transform a digital core device or transistor digital power supply (DVDD) voltage signal to a high power device or transistor power supply (VH) voltage signal. FIG. 1 shows a conventional level shifter for transforming a DVDD voltage signal to a VH voltage signal. As shown, the level shifter includes an input stage 101 and an output stage 103. The input stage receives an input voltage Vin and generates an output voltage Vout. Vin is a DVDD voltage signal having a voltage range of about 0-0.95V while Vout is a VH voltage signal having a voltage range of about 0-3.5V.
The various stages of the level shifter employ 6V high voltage devices with a VH supply source. Devices typically have a minimum ‘turn on’ threshold voltage (Vth) of about 15% of the supply voltage, i.e., for 6V high voltage devices, this equates to approximately 0.9V. As such, Vin has about a 5% margin to switch on the 6V devices. Taking into consideration temperature fluctuation as well as process variations, such a small margin can lead to situations where the high voltage devices fail to switch on, rending the level shifter inoperable.
From the foregoing, it is desirable to provide an improved level shifter for transforming a low voltage range signal to a high voltage range signal.